Semiconductor device with stepped well

ABSTRACT

A semiconductor device comprises a first conductivity type semiconductor layer and a second conductivity type well region which is formed on the semiconductor layer. The well region includes a first semiconductor region of a first depth and a second semiconductor region of a second depth deeper than the first depth which is provided in the central portion of the first semiconductor region. The ratio of the first depth to the second depth is settled in a range from 0.85 to 0.95 in order to increase the breakdown voltage of the semiconductor device.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device and a method of fabricating the same and, more particularly, it relates to a semiconductor device such as a power MOSFET and IGBT (Insulated gate bipolar transistor) with increased breakdown voltage and a method of fabricating the same.

2. Description of the Background Art

FIG. 1 is a sectional view showing a plurality of basic MOS unit cells of a conventional n channel power MOSFET device.

Referring to FIG. 1, an n⁻ drain region 1b is superposed on the upper surface of an n⁺ drain region 1a. A plurality of separate p semiconductor regions 2 are selectively formed in the surface of the n⁻ drain region 1b. N⁺ source regions 3 are selectively formed in the surfaces of the p semiconductor regions 2, each of which is spaced by a predetermined interval from the n⁻ drain region 1b. Channel regions 4 are defined near portions of the surfaces of the p semiconductor regions 2 between the n⁺ source regions 3 and the n⁻ drain region 1b. Gate insulation films 5 are formed on the channel regions 4, and gate electrodes 6 are superposed thereon. Further, a source electrode 7 is formed to connect and short-circuit the central portions of the surfaces of the p semiconductor regions 2 and parts of the surfaces of the n⁺ source regions 3. The gate electrodes 6 and the source electrode 7 are electrically isolated by layer insulation films 8 interposed therebetween. A drain electrode 9 is formed on the bottom surface of the n⁺ drain region 1a.

An operation of the conventional semiconductor device in FIG. 1 will now be described. A drain voltage V_(DS) is applied across the drain electrode 9 and the source electrode 7. When a gate voltage V_(GS) is applied across the gate electrodes 6 and the source electrode 7, inversion layers are formed in the channel regions 4. A drain current I_(D) flows between the drain electrode 9 and the source electrode 7 through the channel regions 4. The drain current I_(D) is controlled by the gate voltage V_(GS). A potential at the channel regions 4 are fixed by connecting and short-circuiting the central poritons of the surfaces of the p semiconductor regions 2 and the parts of the surfaces of the n⁺ source regions 3 with the source electrode 7.

The power MOSFET is liable to break down in such a manner as will be described below. FIG. 2 is a graph illustrating an output characteristic of the power MOSFET shown in FIG. 1. The axis of ordinate of the graph represents the drain current I_(D) and the axis of abscissa represents the drain voltage V_(DS). A parameter is the gate voltage V_(GS). When the drain voltage V_(DS) reaches the breakdown voltage V_(C), the drain current I_(D) rapidly increases to cause the breakdown state of the power MOSFET device. The power MOSFET device tends to be instantaneously destroyed when the breakdown current J_(C) becomes large to the extent of exceeding a certain critical value.

FIG. 3A is a schematic sectional view showing a power MOSFET and FIG. 3B is a similar schematic sectional view of the power MOSFET to which a diagram of its equivalent circuit is superposed. Referring to FIG. 3A, there are internal resistances R₁ and R_(a) near the respective n⁺ source regions 3 in the p semiconductor region 2. The internal resistance R₁ extends in a direction corresponding to the depth of each n⁺ source region 3 while the internal resistance R_(a) extends in a direction along the bottom surface of each n⁺ source region 3. In FIG. 3B, these internal resistances are replaced with a composite internal resistance R₂ extending in a direction corresponding to the depth of the n⁺ source regions 3 and the internal resistances R_(a) extending in a direction along the bottom surfaces of the n⁺ source regions 3 in the p semiconductor region 2. The internal resistances R_(a) serve as a base resistance of a parasitic transistor T_(r) composed of the n⁻ drain region 1b, the p semiconductor region 2 and the n⁺ source regions 3. The n⁻ drain region 1b and the p semiconductor region 2 form a diode D.

When the drain voltage V_(DS) applied across the source electrode 7 and the drain electrode 9 is increased and reaches the breakdown voltage of the diode D defined by the n⁻ drain region 1b and the p semiconductor region 2, the breakdown current J_(C) beings to flow as indicated with arrows in FIG. 3A. When the breakdown current J_(C) flows just under the bottom surfaces of the n⁺ source regions 3, the base potential of the parasitic transistor T_(r) rises. The parasitic transistor T_(r) becomes conductive when the potential difference between the base and the emitter becomes more than 0.6 V. This condition is given by the following equation:

    J.sub.C ×R.sub.a >0.6 (V)                            (1)

It should be noted that the composite internal resistance R₂ extending in the direction corresponding to the depth of the n⁺ source regions 3 is sufficiently smaller than the internal resistances R_(a) and therefore it is negligible. When the breakdown current J_(C) which satisfies the equation (1) flows into the transistor T_(r), it becomes conductive.

At this time, the collector current flowing in the parasitic transistor T_(r) is equal to the product of the base current and a direct current amplification factor h_(FE) of the parasitic transistor T_(r). Usually, the value of the direct current amplification factor h_(FE) is very large, and hence the collector current flowing in the parasitic transistor T_(r) is also very large. Thus, when a breakdown voltage is lower in some part than in other parts, no matter how slight the difference may be, the flow of current concentrates in such area, ultimately resulting in breaking down the power MOSFET.

A conventional semiconductor device such as a power MOSFET structured as has been described has an insufficient to breakdown voltage, and therefore it tends to be instantaneously destroyed due to overload.

SUMMARY OF THE INVENTION

A semiconductor device in accordance with the present invention comprises a first conductivity type semiconductor layer and a second conductivity type well region formed on the semiconductor layer. The well region has a first semiconductor region of a first depth and a second semiconductor region of a second depth deeper than the first depth which is provided in a central portion of the first semiconductor region. A ratio of the first depth to the second depth is selected in a range from 0.85 to 0.95.

A method of fabricating a semiconductor device in accordance with the present invention comprises the steps of preparing a first conductivity type semiconductor layer having first and second major surfaces, forming a second conductivity type well region selectively in the first major surface of the semiconductor layer by double diffusion, the well region having a stepped configuration composed of a peripheral portion of a first depth and a central portion of a second depth deeper than the first depth, a ratio of the first depth to the second depth ranging from 0.85 to 0.95, forming a first conductivity type semiconductor region selectively in a surface of the well region, forming a first main electrode on the semiconductor region, and forming a second main electrode on the second major surface of the semiconductor layer. The step of forming a second conductivity type well region includes the step of forming a mask for the double diffusion which includes the steps of forming a patterned insulation layer on the first major surface of the semiconductor layer and forming a patterned control electrode on the insulation layer.

According to the present invention, a well region is formed by a first semiconductor region having a first relatively shallow depth and a second semiconductor region having a second relatively deep depth which is provided in the center of the first semiconductor region in a manner that the ratio of the first depth to the second depth ranges from 0.85 to 0.95. Consequently, a semiconductor device which can not be easily broken down due to overload and a high breakdown voltage can be implemented.

Accordingly, it is an object of the present invention to provide a semiconductor device which can not be easily broken down due to overload and has a high breakdown voltage.

These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a conventional semiconductor device;

FIG. 2 is a graph illustrating an output characteristic of the semiconductor device shown in FIG. 1;

FIGS. 3A and 3B are diagrams for explaining the problems with the semiconductor device shown in FIG. 1;

FIG. 4 is a sectional view showning an embodiment of a semiconductor device according to the present invention;

FIGS. 5A to 5E are sectional views illustrating manufacturing steps of the semiconductor device shown in FIG. 4;

FIGS. 6 to 9 are graphs for explaining an operation of the semiconductor device shown in FIG. 4;

FIG. 10 is a sectional view showing an example of application of the present invention;

FIG. 11 is a sectional view showing another embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 4 is a sectional view showing a basic MOS unit cell of an n channel power MOSFET which is an embodiment of a semiconductor device according to the present invention. In this embodiment, a p semiconductor (well) region 2 is composed of a relatively shallow first semiconductor region 2a and a relatively deep second semiconductor region 2b provided in the center of the first semiconductor region 2a. The second semiconductor region 2b is provided for forcing the breakdown current J_(C) to flow only in this part.

Other structures of this power MOSFET are the same as those in the above mentioned conventional power MOSFET. That is, an n⁻ drain region 1b is formed on an n⁺ drain region 1a. A plurality of the separate p semiconductor regions 2 composed of the first and second semiconductor regions 2a, 2b are selectively provided in the surface of the n⁻ drain region 1b. In the surfaces of the p semiconductor regions 2, n⁺ source regions 3 are selectively formed at predetermined intervals from the n⁻ drain region 1b. The n⁺ source regions 3, for example, may be annular. Channel regions 4 are defined in the vicinity of the surfaces of the p semiconductor regions 2 between the surfaces of the annular n⁺ source regions 3 and the n⁻ drain region 1b. Gate insulation films 5 are formed on the channel regions 4 and gate electrodes 6 are superposed thereon. Additionally, a source electrode 7 is formed in common with all the unit cells to connect and short-circuit the central portions of the surfaces of the p semiconductor regions 2 and parts of the surfaces of the n⁺ source regions 3. The gate electrodes 6 and the source electrode 7 are electrically isolated by layer insulation films 8 interposed therebetween. A drain electrode 9 is formed on the rear surface of the n⁺ drain region 1a.

The basic operation of this n channel power MOSFET is the same as that of the aforementioned conventional power MOSFET as well be described below. A drain voltage V_(DS) is applied across the drain electrode 9 and the source electrode 7. When a gate voltage V_(GS) is applied across the gate electrodes 6 and the source electrode 7, inversion layers are formed in the channel regions 4, so that drain current I_(D) flows between the drain electrode 9 and the source electrode 7 through the channel regions 4. The drain current I_(D) is controlled by the gate voltage V_(GS). A potential at the channel regions 4 is fixed by connecting and short-circuiting the central portions of the surfaces of the p semiconductor regions 2 and parts of the surfaces of the n⁺ source regions 3 with the source electrode 7.

Now, the manufacturing steps of this n channel power MOSFET will be described. FIGS. 5A to 5E are sectional views illustrating an example of a method for manufacturing the n channel power MOSFET shown in FIG. 4.

Referring to FIG. 5A, the n⁻ drain region 1b is deposited on the n⁺ drain region 1a by means of epitaxial growing. An oxide film is deposited on the n⁻ drain region 1b and patterned by means of photolithography to form the gate oxide films 5. A p type impurity 100 such as boron is injected to the n⁻ drain region 1b through a mask of the gate oxide films 5 by means of ion implantation. After that, a heat treatment at approximately 1200° C. for a specific period of time causes the p type impurity 100 to diffuse up to several μm in depth so that p semiconductor regions 20 are formed as shown in FIG. 5B.

Then, polysilicon 6a is deposited and thereafter both the polysilicon 6a and the gate oxide films 5 are patterned by means of photolithography so as to open windows therein which are larger than those patterned in the course of the previous step. The p type impurity 100 is injected again to the n⁻ drain region 1b through a mask of the polysilicon 6a by means of ion implantation. After that, the p semiconductor regions 2 each of which is composed of the relatively shallow first semiconductor region 2a and the relatively deep second semiconductor region 2b provided in the center portion of the first semiconductor region 2a are formed by means of a heat treatment at approximately 1200° C. for a specific period of time, as shown in FIG. 5C. Adjusting a time period for annealing after ion implantation in each step can vary depth of these semiconductor regions 2a and 2b.

An n type impurity 200 such as phosphorus is deposited to the p semiconductor regions 2 through the mask of the polysilicon 6a, and thereafter the shallow n⁺ source regions 3 are formed by means of a heat treatment at 900° C. for a specific period of time. Upon formation of these regions 3, an n type impurity 200 such as phosphorus comes into the polysilicon 6a and thus the ploysilicon 6a, being thereby conductive, is developed into the gate electrode 6 as shown in FIG. 5D.

Referring to FIG. 5E, an oxide film is deposited and then holes are made in contact portions by means of photolithography to form the layer insulation films 8. After parts of n⁺ source regions 3 are eliminated by etching, the source electrode 7 is formed by depositing aluminum on the layer insulation films 8 and openings therein. Further, the drain electrode 9 is formed on the rear surface of the n⁺ drain region 1a.

This power MOSFET has an increased breakdown voltage as will be described below. Each p semiconductor (well) region 2 of this power MOSFET is formed by the relatively shallow first semiconductor region 2a and the relatively deep second semiconductor region 2b provided in the center portion of the first semiconductor region 2a, so that the flow of the breakdown current J_(C) concentrates in the relatively deep second semiconductor regions 2b, especially near the center portion in the second semiconductor regions 2b. As a result, since the current flowing just under the n⁺ source regions 3 is reduced, a parasitic transistor T_(r) can not be easily activated, or its collector current becomes sufficiently small even when it is activated. The paratstic transistor T_(r), which cannot be thus easily triggered, does not cause a breakdown easily even in the case of overload.

The flow of the breakdown current J_(C) concentrates in the relatively deep second semiconductor regions 2b for the following reasons. As has been described, the second semiconductor regions 2b are formed by means of thermal diffusion. Then, the bottom surfaces of the second semiconductor regions 2b are somewhat spherical but not entirely flat as shown in FIG. 4. Accordingly, the radius of curvature of each depletion layer extending from the boundary between the n⁻ drain region 1b and each p semiconductor region 2 in applying the drain voltage V_(DS) across the source electrode 7 and the drain electrode 9 is smaller in a part corresponding to each P bottom surface of the second semiconductor region 2b compared with the case in which no second semiconductor region 2b is provided. As a result, electric field is liable to concentrate, i.e., become strong in the bottom surfaces of the second semiconductor regions 2b, so that the flow of the breakdown current J_(C) concentrates in the second semiconductor regions 2b. This is one reason.

Another reason relates to the thickness of the n⁻ drain region 1b. Specifically, the n⁻ drain region 1b has the minimum thickness in a part A shown in FIG. 4 because of the existence of the relatively deep second regions 2b. The thinness of the n⁻ drain region 1b means a small resistance in that part. Thus, current is apt to flow most in the part A, so that the flow of the breakdown current J_(C) concentrates in the semiconductor regions 2b. For the above stated reasons, the flow of the breakdown current J_(C) concentrates in the second semiconductor regions 2b, so that a voltage drop generated by an internal resistor R_(a) (see FIG. 3B) becomes less and the parasitic transistor T_(r) (see FIG. 3B) therefore cannot be easily triggered.

Now, what effect the ratio of the depth x₁ of the first semiconductor region 2a to the depth x₂ of the second semiconductor region 2b has on the breakdown voltage will be discussed. Such analysis has not been attempted and the value x₁ /x₂ has generally been determined approximately at 0.5-0.7 with the aim of preventing a trigger of the paratsitic transistor T_(r) (see FIG. 3B). As a result of a simulation for examining a relationship between the value of x₁ /x₂ and the breakdown voltage, however, it has been proved that the breakdown voltage reaches the maximum when the value of x₁ /x₂ is approximately 0.9. A brief outline of the simulation and its results will be described below.

A program for the simulation includes steps of dividing a cell region of the power MOSFET into a mesh, solving the Poisson equation according to data on an impurity distribution and an applied voltage of each mesh element to obtain a distribution of electric field in the cell region and finding an amplification factor of an electron or hole avalanche from the obtained distribution of the electric field to know the breakdown voltage. As the above mentioned data on the impurity distribution, the result of a simulation performed according to the simulation program for evaluating an impurity profile was utilized.

An example of numerical calculation will be described with regard to the power MOSFET of 500 V class. In order to attain the breakdown voltage of 550 V, for example, the n⁻ drain region 1b must have an impurity concentration C_(B) of 3.3×10¹⁴ cm⁻³ or below and the thickness of 34 μm or more on the premise that the n⁻ drain region 1b and the p semiconductor region 2 make an ideal plane junction extending infinitely. In many cases, an actual device has a curved interface in its pn junction. Electric field locally concentrates in the curved portion and therefore the breakdown voltage is lower compared with the case of a plane junction. Accordingly, a problem to be solved in a device design is how the breakdown voltage can be approached to that in the case of a plane junction.

In this simulation, an interval B between the adjacent p semiconductor regions 2 is used as a parameter, and the depth x₂ is varied under the condition that x₁ =4.5 μm and the width C of the first semiconductor regions 2a is settled at 4.5 μm. Further, the data on the thickness of the n⁻ drain region 1b is settled to 45 μm before the formation of the p semiconductor regions 2, allowing for the thickness being reduced due to the p semiconductor regions 2.

As a result of the simulation in the case of the plane junction, it is proved that the breakdown voltage is 565 V when the thickness of the p semiconductor regions 2 is settled to 6 μm. The following Tables 1A to 1E exhibit a simulation breakdown voltage V_(BR) and the ratio of the simulation breakdown voltage V_(BR) to the simulation plane junction breakdown voltage (565 V) in the case that the depth x₂ is varied. Moreover, the following Table 2 exhibits the ratio of the depth x₁ to the depth x₂.

                  TABLE 1A     ______________________________________     B = 1 μm     x.sub.2 (μm)                  4.5    5.2         6.0  6.8     ______________________________________     V.sub.BR (V) 550    530         511  --     V.sub.BR /565                   0.97   0.94        0.90                                          --     ______________________________________

                  TABLE 1B     ______________________________________     B = 5 μm     x.sub.2 (μm)                  4.5    5.2         6.0  6.8     ______________________________________     V.sub.BR (V) 535    515         499  474     V.sub.BR /565                   0.95   0.91        0.88                                          0.84     ______________________________________

                  TABLE 1C     ______________________________________     B = 9 μm     x.sub.2 (μm)                  4.5    5.2         6.0  6.8     ______________________________________     V.sub.BR (V) 515    530         490  460     V.sub.BR /565                   0.91   0.94        0.87                                           0.81     ______________________________________

                  TABLE 1D     ______________________________________     B = 13 μm     x.sub.2 (μm)                  4.5    5.2         6.0  6.8     ______________________________________     V.sub.BR (V) 502    518         482  453     V.sub.BR /565                   0.89   0.92        0.85                                           0.80     ______________________________________

                  TABLE 1E     ______________________________________     B = 17 μm     x.sub.2 (μm) j                  4.5    5.2         6.0  6.8     ______________________________________     V.sub.BR (V) 492    508         463  447     V.sub.BR /565                   0.87   0.90        0.82                                           0.79     ______________________________________

                  TABLE 2     ______________________________________     x.sub.1 = 4.5 μm     x.sub.2 (μm)                 4.5   5.2         6.0  6.8     ______________________________________     x.sub.1 /x.sub.2                 1      0.87        0.75                                         0.66     ______________________________________

FIG. 6 is a graph illustrating a relationship between x₁ /x₂ and V_(BR) /565 based upon the simulation result of the Tables 1A to 1E. The graph of FIG. 6 should be taken as follows:

(1) In the case that B=9 μm, 13 μm or 17 μm, the simulation breakdown voltage V_(BR) becomes closest to the simulation plane junction breakdown voltage (565 V) when the value of x₁ /x₂ is approximately 0.9. That is, the simulation breakdown voltage V_(BR) reaches the maximum value when x₁ /x₂ is approximately 0.9. This is because the best connection of the depletion layers between parts I and II in the p semiconductor regions 2 is provided when the value of x₁ /x₂ is approximately 0.9, so that the concentration of electric field may occur in the least degree.

(2) In the case that B=9 μm, 13 μm or 17 μm, the simulation breakdown voltage V_(BR) is higher than the breakdown voltage in the case that no second semiconductor region 2b is provided (i.e., x₁ /x₂ =1), in an approximate range that 0.81<x₁ /x₂ <1.

Table 3 exhibits a relationship between B/x₁ and V_(BR) /565 when x₂ =5.2 μm, x₁ /x₂ =0.9 and the width C of the second semiconductor regions 2a is equal to x₁.

                  TABLE 3     ______________________________________     B (μm)              1         5      9       13   17     ______________________________________     B/x.sub.1              0.21      1.06   1.92    2.78 3.63     V.sub.BR /565              0.94      0.91   0.94    0.92 0.90     ______________________________________

The relation between B/x₁ and V_(BR) /565 can be represented in a graph as in FIG. 7.

The graph of FIG. 7 should be taken as follows: As the interval B varies, the breakdown voltage also varies. The breakdown voltage reaches the minimum value when B/x₁ ≈0.7-0.9 and reaches the maximum value when B/x₁ ≈2. In a range that B/x₁ >2, the breakdown voltage decreases gradually and monotonously in accordance with increase of the interval B. With regard to a power MOSFET, ON current flows in the n⁻ drain region 1b between the p semiconductor regions 2, and therefore it is desirable that the interval B is large for reducing ON resistance. Thus, to keep the value of V_(BR) /565 large and the reduce ON resistance, there is practically no problem when the value of the interval B satisfies the condition that B/x₁ >2, i.e., B>x₁ ×2.

A power MOSFET was experimentaly manufactured based upon the above described simulation results, and the breakdown voltage therein was measured. The results of the experiment will be described below.

Table 4 exhibits an experimental breakdown voltage V_(BR) ' and the ratio of the experimental breakdown voltage V_(BR) ' to an experimental plane junction breakdown voltage (525 V) in the case that the depth x₂ is varied under the condition that x₁ =4.5 μm, B=10 μm and an impurity concentration C_(B) in the n⁻ drain region 1b is equal to 3.3×10¹⁴ cm⁻³. Table 5 exhibits values of x₁ /x₂. The condition in which the experimental plane junction breakdown voltage (525 v) is measured is that the p semiconductor regions 2 are 6 μm in thickness, the n⁻ drain region 1b is 45 μm in thickness and the n⁻ drain region 1b is 3.3×10¹⁴ cm⁻³ in its impurity concentration.

                  TABLE 4     ______________________________________     x.sub.2 (μm)                4.7          5.2    6.0     ______________________________________     V.sub.BR ' ( V )                470          490    440     V.sub.BR '/525                 0.90         0.93   0.84     ______________________________________

                  TABLE 5     ______________________________________     x.sub.1 = 4.5 μm     x.sub.2 (μm)               4.7           5.2    6.0     ______________________________________     x.sub.1 /x.sub.2                0.95          0.87   0.75     ______________________________________

FIG. 8 illustrates the experiment results in a graph by normalizing the results appearing in Table 3. FIG. 8 should be taken as follows:

(3) The experimental breakdown voltage V_(BR) ' is made closest to the experimental plane junction breakdown voltage (525 V) when the value of x₁ /x₂ is approximately 0.89. That is, the experimental breakdown voltage V_(BR) ' reaches the maximum value when x₁ /x₂ is approximately 0.89.

(4) The experimental breakdown voltage V_(BR) ' is higher than the breakdown voltage in the case that no second semiconductor region 2b is provided (i.e., x₁ /x₂ =1), in a range that 0.78<x₁ /x₂ <1.

The above (3) and (4) are almost the same as the aforementioned (1) and (2) given by the simulation results. From these experimental results and simulation results, it should be noted that higher breakdown voltage can be obtained when 0.85<x₁ /x₂ <0.95.

Although x₁ =4.5 μm in the above embodiment, x₁ may take any value if x₁ /x₂ is within a range from 0.85 to 0.95.

Now, a relationship between the width C of the first semiconductor regions 2a and the breakdown voltage will be described. When the width C is inappropriately large, good connection in the depletion layers between the portions I and II in the p semiconductor regions 2 is not obtained, and therefore a concentration of electric field occurs in this portion and the breakdown voltage goes down. When C=6.5 μm under the condition that x₁ =4.5 μm, x₂ =5.2 μm and B=9 μm, the simulation breakdown voltage V_(BR) was 521 V and the experimental breakdown voltage V_(BR) ' was 480 V. On the other hand, when C=4.5 μm in the same condition, the simulation breakdown voltage V_(BR) was 530 V (see Table 1C), and when C=4.5 μm under the condition that x₁ =4.5 μm, x₂ =5.2 μm and B=10 μm (take it ≃ 9 μm), the experimental breakdown voltage V_(BR) ' is 490 V (see Table 4). A relationship between the above mentioned simulation breakdown voltage V_(BR) or experimental breakdown voltage V_(BR) ' and the width C is shown in FIG. 9. As can be seen in FIG. 9, both the simulation breakdown voltage V_(BR) and the experimental breakdown voltage V_(BR) ' are conjectured to have a linear relationship with the width C. When no second semiconductor region 2b is provided, the simulation breakdown voltage V_(BR) is 515 V (see Table 1C) assuming that, for example, x₁ =x₂ =4.5 μm, and then the width C is approximately 7.9 μm as will be recognized from FIG. 9. When x₁ =4.5 μm and x₂ =4.7 μm, which is equivalent to the case that no second semiconductor region 2b is provided, the experimental breakdown voltage V_(BR) ' is 470 V, and then the width C is approximately 8.4 μm as will be recognized from FIG. 9. Thus, the breakdown voltage becomes larger under the condition that C≦2x₁, compared with the case that no second semiconductor region 2b is provided. It should be noted that higher breakdown voltage can be obtained compared with the case that no second semiconductor region 2b is provided if the relationship between the breakdown voltage and the width C desirably satisfies C≦1.5x₁.

In the above embodiment, p semiconductor regions, or a well regions are defined to have two steps in depth. As shown in FIG. 10, however, a p guard ring around a chip in which a p well region is formed may be defined on an n semiconductor layer to have several steps in depth so that the ratio of a depth between adjacent steps (i.e., x_(a) /x_(b), x_(b) /x_(c), x_(c) /x_(d)) is settled in a range from 0.85 to 0.95. In this way, concentration of electric field around the chip can be relieved and higher breakdown voltage can be maintained. The concentration of electric field around the chip can be further relieved by satisfying the condition that C₁ ≦2x_(c), C₂ ≦2x_(b), C₃ ≦2x_(a).

FIG. 11 is a sectional view showing an IGBT to which the present invention is applied. Referring to FIG. 11, there is a structural difference from the power MOSFET shown in FIG. 4, that is, a p⁺ region 1c is further formed between an electrode 9 and an n⁺ region 1a. Other parts, that is, the n⁺ region 1a, an n⁻ region 1b, p semiconductor regions 2, n⁺ regions 3, gate insulation films 5, gate electrodes 6, an electrode 7, layer insulation films 8 and the electrode 9 are the same as in the power MOSFET shown in FIG. 4. Thus, the present invention is applicable to an IGBT structured similarly to the above mentioned power MOSFET. The present invention is applicable to semiconductor devices such as a power MOSFET and an IGBT as well as other semiconductor devices structured similar to those semiconductor devices.

Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation. The spirit and scope of the present invention should be limited only by the terms of the appended claims. 

What is claimed is:
 1. A semiconductor device comprising:a first conductivity type semiconductor layer; and a second conductivity type well region formed on said semiconductor layer; said well region having a first semiconductor region of a first depth and a second semiconductor region of a second depth deeper than said first depth, said second semiconductor region being provided in a central portion of said first semiconductor region, a ratio of said first depth to said second depth ranging from 0.85 to 0.95.
 2. A semiconductor device in accordance with claim 1, whereina width of said first semiconductor region is less than or equal to 1.5 times of said first depth.
 3. A semiconductor device in accordance with claim 1, whereinsaid well region includes a plurality of well regions, and a distance between said plurality of well regions is greater than two times of said first depth.
 4. A semiconductor device comprising:a first conductivity type semiconductor layer; a second conductivity type well region formed on said semiconductor layer; and a second conductivity type guard ring formed on said semiconductor layer around said well region, said guard ring having a multistep structure in which an outer step of said multistep structure is shallower in depth than an inner step of said multistep structure and a ratio of a depth between adjacent steps of said multistep structure is 0.85 to 0.95.
 5. A semiconductor device comprising:a first conductivity type semiconductor layer having first and second major surface; a second conductivity type well region selectively formed in said first major surface of said semiconductor layer; a first conductivity type semiconductor region selectively formed in a surface of said well region; an insulation layer formed on a portion of a surface of said well region between surfaces of said semiconductor layer and said semiconductor region; a control electrode formed on said insulation layer; a first main electrode formed on said semiconductor region; and a second main electrode formed on said second major surface of said semiconductor layer, said well region having a steped configuration composed of a peripheral portion of a first depth and a central portion of a second depth deeper than said first depth, a ratio of said first depth to said second depth ranging from 0.85 to 0.95.
 6. A semiconductor device comprising:a first conductivity type first semiconductor layer having first and second major surface; a second conductivity type well region selectively formed in said first major surface of said first semiconductor layer; a first conductivity type semiconductor region selectively formed in a surface of said well region; a second conductivity type second semiconductor layer formed on said second major surface of said first semiconductor layer; an insulation layer formed on a portion of a surface of said well region between surfaces of said first semiconductor layer and said semiconductor region; a control electrode formed on said insulation layer; a first main electrode formed on said semiconductor region; and a second main electrode formed on said second semiconductor layer, said well region having a steped configuration composed of a peripheral portion of a first depth and a central portion of a second depth deeper than said first depth, a ratio of said first depth to said second depth ranging from 0.85 to 0.95. 